Chapter 8 Typecasting and Inheritance 1 Type Casting

Chapter 8 Typecasting and Inheritance 1

Type Casting z. Allows flexible structures to be built z. Idea: Treat a derived class object as if it were a base class object z. Syntax: This syntax cannot be used with pointers. Base. Class. Name (derived. Class. Object) or (Base. Class. Name) derived. Class. Object 2

Type Casting Displayable. Number* Show. In Show Value Number* Next 3

Type Casting Using Pointers to an Object Text. Box display(Location(100, 100), Shape(75, 50)); Number* number = new Number(100); Displayable. Number* dnp; Number->Next(); Type Cast dnp = (Displayable. Number*) number; dnp->Show. In(display); dnp->Show(); We may only call methods from Displayable. Number on dnp, even though it’s really an object of type Number! 4

Type Casting Using References Text. Box display(Location(100, 100), Shape(75, 50)); Number number(100); number. Next(); Displayable. Number& displayable = (Displayable. Number&) number; displayable. Show. In(display); displayable. Show(); 5

Type Casting Errors Displayable. Number* num. Ptr; Number *count 1 = new Number(100); Number count 2(200); num. Ptr = (Displayable. Number*) count 1; Displayable. Number& num. Ref = (Displayable. Number)count 2 num. Ptr->Next(); num. Ref. Next(); Displayable. Number doesn’t have a Next() method! 6

Why Type Casting? z. Allows us to treat a collection of objects uniformly by viewing them as their base class. z. Example 1 : Shapes in a Graphical Editor z. Example 2 : Number. Panel class example 7

Shapes z. Derive Circle and Rectangle from “Shape” yavoid code duplication y. Shape. Manager handles instances of Shape y. E. g. : Location loc Rectangle bounds Set. Location Get. Bounds. . . Game. Circle Game. Rectangle Set. Center Set. Radius Draw. . . Set. Size Draw. . . 8

Type Casting to Create a Polymorphic Structure class Number. Panel { private: Displayable. Number *number[3]; // array for simplicity int last; Panel *panel; public: Number. Panel(); void Add( Displayable. Number* num ); // add num void Show. In(Panel & panel); // put all objects here void Show(); // Show all objects in panel }; 9

Implementation of the Number. Panel Class Number. Panel: : Number. Panel() { last = -1; panel = (Panel *)0; } void Number. Panel: : Add( Displayable. Number* num) { if ( last < 2 ) { number[++last] = num; Text. Box* tbox = new Text. Box(Location(last*60, 20) Shape(50, 20)); num->Show. In(*tbox); if (panel) panel->Add(*tbox); } 10 }

Implementation of Number. Panel Class (Cont. ) void Number. Panel: : Show. In( Panel& panel) { this->panel = &panel; } void Number. Panel: : Show() { if (last > -1 ) for(int i = 0; i <= last; i++) number[i]->Show(); } Number. Panel: : ~Number. Panel() {} 11

Example of Number. Panel Flexibility Frame display(Location(100, 100), Shape(200, 200)); Panel panel(display, Location(10, 10), Shape(180, 40)); Number. Panel number. Panel; Number number(100); Cycler octal(8); Jump. Counter length(0); number. Panel. Show. In(panel); // add different kinds of counters to the panel number. Panel. Add((Displayable. Number*)&number); number. Panel. Add((Displayable. Number*)&cycler); number. Panel. Add((Displayable. Number*)&octal); 12

Example of Number. Panel Flexibility (Continued) // manipulate individual counter objects number. Next(); octal. Next(); length. Next(50); // display all of the new values number. Panel. Show(); 13

Implicit vs. Explicit Type Casting Explicit Number *number; Displayable. Number *displayable =(Displayable. Number*)number Implicit Number *number; Displayable. Number *displayable = number; Avoid implicit typecasting. Say what you mean in your code! 14

Implicit vs. Explicit Type Casting In Parameter Passing Given the following method: Number. Panel: : Add(Displayable. Number * dn); Call with Explicit Type Cast Number. Panel panel; Number *n = new Number(100); panel. Add((Displayable. Number*)n); Call with Implicit Type Cast Number. Panel panel; Number *c = new Number(100); panel. Add(c); Other developers may not realize that Add takes a Displayable. Number 15

Widening vs. Narrowing z. Widening y. Type casting from a derived class to a base class. y. Always Safe! Can be checked by the compiler. z. Narrowing y. Type casting from a base class to a derived class. y. Safety depends on the programmer. 16

Widening/Narrowing Example Displayable. Number *DNptr; Number *number = new Number(100); Cycler *cycler; DNptr = (Displayable. Number*)number; // safe; it widens cycler = (Cycler*)DNptr; cycler->Next(); // oops! // who knows what this will do! 17